Up and to the light: intra- and interspecific variability of photo- and geo-tactic oviposition preferences in genus Trichogramma

Trichogramma are parasitic microwasps much used as biological control agents. The genus is known to harbor tremendous diversity, at both inter- and intra-specific levels. The successful selection of Trichogramma strains for biocontrol depends on characterizing the existing diversity in this group, especially regarding oviposition performance and behavior. Oviposition preferences in relation to environmental cues such as light and gravity have received little attention so far, although they are suspected to play an important role in microhabitat selection and parasitism patterns. The extent of their variability, and their potential correlated variations, is virtually unknown. Here we use a novel experimental approach relying on automatic image analysis to characterize the oviposition preferences in relation to light and gravity, as well as their interaction, in 25 populations of Trichogramma from five species. We show that most Trichogramma populations and species harbour preferences for light and preferences for elevated parts. However, the two traits harbor significant inter and intraspecific variation. The effects of light and gravity on oviposition patterns were found to be almost perfectly additive overall, with two exceptions. Oviposition preference patterns were not static but very plastic in time: preferences tended to relax over consecutive days, and the strongest preferences relaxed the fastest, presumably because of the density-dependent effect of resource depletion. A correlation of oviposition patterns with the vegetation stratum at which populations were sampled suggests that different species/populations may be associated with different strata with corresponding differentiation in light- and gravity-related oviposition preferences.